Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02419220 2003-02-18
The invention relates to a device for separating solids from liquids by
means of flotation, particularly for separating impurities and ink particles
from fibre suspensions, with several injectors, which are provided inside a
flotation cell.
s Deinking flotation is a mechanical process for separating impurities and
ink particles from fibre suspensions, particularly those produced in waste
paper treatment. This process is linked to generating a suitable amount of
gas bubbles over an appropriate range of bubble sizes. By changing the
gas-liquid flows and the distribution of bubble sizes, ores, etc. can also be
1o separated from low-grade, unusable material. A similar principle is
applied in solid-liquid separation, e.g. in waste water.
Hydrophobic solids or such solids as ink particles or stickies (e.g. also coal
as accept material) that have been made hydrophobic by applying
surface-active substances are carried to the surface of the liquid by the
15 gas bubbles adhering to them and can then be removed there as foam.
In the pulp and paper industry, this method of using self priming injectors
- mainly comprising a propulsion jet nozzle, mixing pipe or impulse
exchange chamber and diffuser - has proved successful in generating
gas bubbles and mixing them into the pulp suspension. These individual
2o diffusers are usually assembled in small, round diffuser units (up to 12
individual diffusers). At the exit from the diffuser used for energy recovery
a fibre stock bubble dispersion forms.
Processes of this type are known in numerous geometric modifications,
e.g. from DE 41 16 916 C2, EP 0 211 834 B1 or AT 408 957, and have
2s reached a high technical standard. Nevertheless, two disadvantages arise
when using this process in large-scale industrial applications:
The suction effect of the diffuser units known suffers negative influence if
there are eight injectors per unit or more because some of the injectors on
the inside have a shorter supply of gas due to the injectors surrounding
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them. As a result of this short supply, there is a lower overall gas loading,
which can lead to the propulsion jet being scattered. This reduces the
operating reliability andlor the desired distribution of bubble sizes widens,
which leads to a drop in efficiency in selective flotation or to higher
losses.
The aim of the invention is thus to put together flotation cells that operate
independently of the number of injectors, with the same suction effect and
optimum bubble size distribution for each individual injector, for use in
flotation, mainly in deinking and waste water applications.
The invention is thus characterised by the injectors being arranged in one
1o row. This provides efficient and easy gas access, which is present to the
same extent for each individual nozzle. This applies to all injectors or
nozzles regardless of the number of suction points.
1f, this Leads to favourable placement of the injectors within the flotation
cell, distributed over the surface area, and thus to a better overall flow
1 s concept.
In an advantageous embodiment the rows are arranged in axial direction
in the filotation cell. As a result, this achieves an even flow of foam over
the entire cell length in the direction of the overfilow weir. Furthermore,
the
accept or pulp to be aerated again can be removed by suction while
20 largely avoiding short circuit flows.
In a flotation cell with a foam channel, the injector rows are arranged on
the side of the flotation cell opposite the foam channel, which ensures
complete flow through the entire flotation cell.
It has proved particularly advantageous if the injector rows are arranged in
2s the middle of the flotation cell if there are two foam channels. This
guarantees even loading of the overflow weirs and foam channels.
The invention will now be described using the examples in the drawings,
where Fig. 1 shows a diagram of how the invention is applied, Fig. 2
shows a variant of the invention, Fig. 3 illustrates how a variant of the
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invention is installed, Fig. 4a contains a flotation cell embodiment
according to the invention with one foam channel and Fig. 4b a flotation
cell embodiment according to the invention with two foam channels, Fig.
4c illustrates an embodiment of a flotation cell according to the invention
with three foam channels, and Fig. 5 shows a further embodiment
according to the invention.
Figure 1 shows a diagram of a flotation unit in which the device according
to the invention is installed. The flotation cell 1 is filled for the most
part
with suspension 2, on the surface 3 of which a layer of foam 4 forms,
containing as large a portion as possible of the impurities and ink particles
to be removed by flotation. The foam 4 can drain off through a foam
channel 5 as overflow U. The fibre stock suspension S enters cell 1
through the nozzle unit (injector bundle) 6. The dispersion of bubbles and
fibre stock leaves the diffuser 9 after impinging on an impact plate 8. The
bubbles adhere to the hydrophobic impurities and carry them to the
surface 3. The cleaned suspension leaves the flotation cell as accept G.
In Fig. 2 a diagram illustrates a possible variant of a line diffuser bundle 6
according to the invention, each with 6 individual injectors 10. In this
case, the number of injectors can be varied and the requirements adapted
2o hydraulically or according to production. If the air is offered to the open
jet
as evenly as possible from all sides at the lowest possible pressure loss in
the first injector part; the line arrangement provides a correspondingly
even distribution of bubbles in the blow-out sector of the injector impact
plate. There is a much lower agglomerating tendency by the bubbles in
2s the impact and blow-out sector of the injector units, and the narrow bubble
size distribution generated can also be retained into the flotation sector
thanks to the more favourable flow path after the mixing chamber, leading
into the impact chamber and blow-out sector. Thus, overall aeration can
be performed in a more favourable bubble size spectrum - avoiding one
3o bubbles but without the risk of larger bubbles agglomerating. The width of
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distribution and the selective setting for optimum coverage of the desired
impurity grain size is thus maintained.
Figure 3 illustrates installation of a device according to the invention as a
line injector bundle 6 in a potation unit 1, where the plant can be operated
at any desired high throughput. The illustration shows two consecutive
cells, n and n+1. By increasing the length of the overflow weirs 11, the
specific overflow speed of the pulp to be flotated can be seduced. In order
to ensure even loading of the overflow weirs 11, the line diffusers 6 are
installed in a central position in relation to the suspension surface 3. The
1o diffuser bundles 6 can be arranged such that the suspension flows into the
flotation chamber from above, from the side, or from below.
Figures 4a, 4b and 4c show a horizontal projection of two consecutive
cells in each case, with different arrangements of injector blocks 6.
Straddling the module diffusers in axial direction requires on the one hand
an even flow of foam over the entire cell length towards the overflow weir
11, and on the other hand, the accept or the stock to be aerated again can
be removed by suction while largely avoiding short-circuit flows. In this
way the fibre stock losses can be reduced. The device according to the
invention is also capable of introducing more air and thus improves
2o removal of impurities. Figure 4a shows a flotation cell 1 with a foam
channel 5, where the diffuser blocks 6 are arranged here at the side 12
facing the overflow channel 5. In flotation cells 1 with two foam channels
5, as shown in Fig. 4b, the diffuser blocks 6 are arranged in the middle so
that there is an even flow of foam on all sides and with short flow paths.
The variant in Fig. 4c shows two parallel flotation cells which have three
foam channels 5, where the two cells can also be mounted one behind the
other, however this is used preferably to increase the throughput as a
parallel arrangement.
Figure 5 now shows an arrangement of the injector blocks 6 in a way that
ao the flow 13 of suspension through the flotation cells is deflected through
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the injector blocks 6 in the direction 13', which thus extends the flow path
and with it, the retention time. Here, one or more injector blocks 6 can be
arranged in one flotation cell 1. In Fig. 5, the broken line 14 indicates that
it is possible to divide individual flotation cells.
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